Study On Soil Heavy Metal Pollution After Ecological Restoration In Oil Shale Mining Area

Oil shale is a solid combustible organic sedimentary rock with a high mineral content.It has been classified as an important alternative energy source in the 21st century due to its resourcefulness and feasibility of exploitation.In recent years,with urban expansion,increasing population density,land use and climate change,the carrying capacity of mining areas has been declining due to the fragility of their ecosystems after restoration,and environmental problems have become prominent.Given that less attention has been paid to the ecological environment of the restored mining area,this paper takes the Maoming oil shale mining area as the research object.Through field investigation,sampling and statistical analysis,comprehensively analyzes the current situation and sources of heavy metal pollution in the surrounding soil of the mining area,the health risks to the surrounding residents,the distribution behavior of heavy metals in different aggregates and their leaching toxicity,and the research results can provide theoretical support and technical guidance for the risk prevention and control of heavy metal pollution in the mining area after ecological restoration.The main results achieved are as follows:The pollution status and sources of heavy metals in soil and surface water around the mining area were revealed.It was found that although the Maoming oil shale mining area has been restored,there is still a certain degree of heavy metal pollution in the soil,which poses a potential threat to the surrounding environment.The concentration of heavy metals in water has little difference,but it was obviously lower than that in soil.There was mild to moderate pollution near the open-pit mine,while the original north-south dump showed moderate to severe pollution.The results of enrichment factors indicated that anthropogenic activities affect the enrichment degree of metals in soil.Geochemical speciation analysis found that As,Cd,Mn,and Pb have high ecological risks.The source apportionment receptor model identified three main metal sources around the mining area,namely,industrial activities,atmospheric deposition and mixed sources,and each source contributed differently to the eight heavy metal elements.Cluster analysis showed that sampling points with similar spatial positions have similar pollution sources.The health risks of oil shale mining area to surrounding residents were evaluated.Based on the bioavailability of heavy metals,a hierarchical health risk model was established to evaluate the non-carcinogenic risk and carcinogenic risk of different populations under three exposure ways.The results showed that children were exposed to significant non-carcinogenic risks through oral ingestion,while the carcinogenic risk for adults through dermal exposure significantly exceeded the acceptable safety level.In terms of spatial distribution,carcinogenic and non-carcinogenic risks accumulate over time during mining,and the risk of exposure to tailings areas remains very high after remediation.Integrating the internal and external factors of heavy metals,a regression tree model based on the bioavailability of heavy metals was established.The results showed that the factors affecting the migration and bioavailability of heavy metals were not only related to the internal factors of heavy metals（such as soil physical and chemical properties and geochemical fractions of heavy metals）,but also influenced by external factors（such as environmental variables,land use and distance from traffic roads）to a great extent,which affected human health in different ways.In a word,the stronger the availability of metals,the greater the environmental health risks.The distribution behavior of heavy metals in soil aggregates under the influence of hydrological factors was analyzed.Due to the high degree of weathering and frequent hydrological and precipitation events in the study area,the dissolution of metals,the desorption of water-soluble ammonium nitrogen and the release of fluoride in the mining area were accelerated.In the tailings-south dump,geological metals,such as Cd,Co,Cr,Ni,and Mn were mainly distributed in 2.36～2.0 mm aggregates,while anthropogenic metals such As Cu,Pb,and As were mainly concentrated in the aggregates of<0.05 mm.Due to the strong nitrification,nitrate nitrogen from the tailings-north dump would pollute the surface water and groundwater in the adjacent area through leaching.Geochemical speciation analysis showed that most of the heavy metals mainly exist in relatively stable iron-manganese oxidation fraction,organic binding fraction and residue fraction.The results of structural equation model revealed that the physicochemical properties of the soil,water-soluble ammonium nitrogen,nitrate nitrogen,and inorganic anions affect the distribution of metals in different aggregate particle sizes.Column experiments found that large aggregates significantly influenced the distribution of most metals in the topsoil,while mineral composition in<0.15mm grain size was more susceptible to change.Overall,the hydrological cycle in coastal mining areas drives the spatial and temporal distribution of heavy metals,thus increasing the uncertainty of their response to risks.The leaching behavior of heavy metals was simulated at different time scales.The results of runoff migration experiments found that Mn and NH₃-N in runoff of tailing surface soil would exceed the minimum water quality standard of surface water and groundwater（Class V water quality）under short-term hydrodynamic conditions,and other indexes such as Cd,Co,Ni,Zn,and F^-were also exceed the standard to varying degrees.The results of batch experiments showed that Mn,Ni,and Zn were the main pollutants in the leaching solution.In the column experiment,the p H of leachate was always in a low range（3.4～4.2）after long-term leaching.However,with the oxidation of sulfide minerals in the column,the concentrations of EC and SO₄^2-in leachate were as high as 4000μS/cm and 3000 mg/L,respectively.In the early stage of leaching（cumulative L/S ratio≤2）,the concentrations of Cd,Pb,Co,Zn,and Ni in the leachate of topsoil significantly exceeded the Class III water quality standard of surface water and groundwater.The relationship between the cumulative release of heavy metals and the cumulative L/S ratio of component solution can be expressed by Langmuir equation.The maximum release of Mn in topsoil and deep soil under equilibrium state was 58.4 mg/kg and 43.4 mg/kg,respectively.After mining,the impact of tailings on the surrounding environment is still significant.Farmland is slightly polluted,the pollution degree of surface water in each section of the basin is increasing year by year,and groundwater is affected by a combination of natural factors and human activities.The results of this study provide theoretical basis and data support for the future risk prevention and control of mining areas and take relevant remedial measures.